Pension and Other Postretirement Benefits (Assumptions) (Details)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Viacom Inc [Member]
Defined Benefit Plan, Assumed Health Care Cost Trend Rates
Ultimate trend rate	5.00%	4.50%
Viacom Inc [Member] \| Pre-65 [Member]
Defined Benefit Plan, Assumed Health Care Cost Trend Rates
Projected health care cost trend rate (pre-65)	6.60%	6.30%
Viacom Inc [Member] \| Post-65 [Member]
Defined Benefit Plan, Assumed Health Care Cost Trend Rates
Projected health care cost trend rate (pre-65)	6.60%	5.70%
Pension benefits [Member]
Weighted average assumptions used to determine benefit obligations at December 31:
Discount rate	2.90%	3.50%	4.50%
Rate of compensation increase	0.00%	3.00%	3.00%
Weighted average assumptions used to determine net periodic costs for the year ended December 31:
Discount rate	3.40%	4.50%	3.80%
Expected long-term return on plan assets	6.40%	6.60%	6.60%
Cash balance interest crediting rate	5.00%	5.00%	5.00%
Rate of compensation increase	3.00%	3.00%	3.00%
Postretirement benefits [Member]
Weighted average assumptions used to determine benefit obligations at December 31:
Discount rate	2.60%	3.30%	4.40%
Weighted average assumptions used to determine net periodic costs for the year ended December 31:
Discount rate	3.30%	4.40%	3.90%
Expected long-term return on plan assets			2.00%
Defined Benefit Plan, Assumed Health Care Cost Trend Rates
Ultimate trend rate	5.00%	5.00%
Postretirement benefits [Member] \| Pre-65 [Member]
Defined Benefit Plan, Assumed Health Care Cost Trend Rates
Projected health care cost trend rate (pre-65)	6.60%	7.00%
Postretirement benefits [Member] \| Post-65 [Member]
Defined Benefit Plan, Assumed Health Care Cost Trend Rates
Projected health care cost trend rate (pre-65)	6.60%	7.00%

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- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

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- Definition

Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

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- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan.

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- Definition

Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(3)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920

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- Definition

Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

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- Definition

Weighted-average interest crediting rate used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, cash balance and other defined benefit plans with promised interest crediting rate.

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- Definition

Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

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- Definition

Ultimate trend rate for health care cost for defined benefit postretirement plan.

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